Spark ignition of an air/fuel mixture within a combustion chamber of an internal combustion engine typically involves igniting the air/fuel mixture with an electric spark jumped between an electrode and a ground electrode of a spark plug. An alternative to spark ignition known in the art is torch jet-assisted spark ignition which, as taught by U.S. Pat. Nos. 3,921,605 to Wyczalek, 4,924,829 to Cheng et al., 5,405,280 to Polikarpus et al., and 5,421,300 to Durling et al., offers several advantages over spark ignition approaches. As the name suggests, torch jet-assisted spark ignition utilizes a jet of burning gases that are propelled into the combustion chamber in order to enhance the burning rate within the combustion chamber by providing increased turbulence as well as presenting a larger flame front area. As a result of a faster burning rate, lower cyclic variation in cylinder pressure is achieved, which enables a higher engine efficiency with a higher compression ratio.
In a torch jet-assisted spark ignition system, the jet typically emanates from a combustion prechamber within the spark plug, passing through an orifice into the main combustion chamber. The axis of the orifice is parallel and often coaxial with the combustion prechamber. Though an air/fuel mixture can be introduced directly into the prechamber through a separate intake valve or fuel injector, it is generally preferable that the air/fuel mixture originates from the main chamber in order to simplify the construction of the engine and its ignition system. Combustion of the air/fuel mixture within the prechamber can be initiated from within by a separate igniter, or initiated by the flame front within the main chamber. With either approach, combustion typically proceeds relatively simultaneously in both the prechamber and the main chamber. However, because of the small relative volume of the prechamber, a high pressure is developed in the prechamber while the pressure is still relatively low in the main chamber. As a result, a jet of burning gases shoots from the prechamber far into the main chamber, significantly increasing the combustion rate in the main chamber.
Engine testing of torch jet spark plugs has verified that torch jet-assisted ignition results in faster burn rates than conventional spark ignition techniques, which produce a fixed flame "kernel" and relies on engine design to achieve suitable flame propagation within the main chamber. Torch jet-assisted ignition also relies on engine design considerations, which include tailoring swirl, turbulence and valve design to control the fuel/air charge for more complete and faster burns. Even with optimal engine design, there are typically regions within a main chamber in which the fuel/air mixture does not burn well, resulting in lower combustion efficiency. Accordingly, further enhancements in combustion efficiency using torch jet-assisted ignition would be desirable, the result of which would provide increased power, reduced emissions and better fuel economy for a given engine design.